Non-contact transmittance photoplethysmographic imaging (PPGI) for long-distance cardiovascular monitoring

TL;DR

This study demonstrates the feasibility of long-distance, non-contact cardiovascular monitoring using a novel transmittance PPGI system with ambient correction, enabling remote heart rate measurement at over a meter distance.

Contribution

The paper introduces the first transmittance PPGI system capable of supermeter-level non-contact cardiovascular monitoring, with innovative hardware, software, and signal processing techniques.

Findings

Significant increase in pulsatile PPGI signal quality after processing.

Statistically significant correlation with ground-truth PPG in short and long-distance settings.

Feasibility of remote heart rate monitoring over a meter distance.

Abstract

Photoplethysmography (PPG) devices are widely used for monitoring cardiovascular function. However, these devices require skin contact, which restrict their use to at-rest short-term monitoring using single-point measurements. Photoplethysmographic imaging (PPGI) has been recently proposed as a non-contact monitoring alternative by measuring blood pulse signals across a spatial region of interest. Existing systems operate in reflectance mode, of which many are limited to short-distance monitoring and are prone to temporal changes in ambient illumination. This paper is the first study to investigate the feasibility of long-distance non-contact cardiovascular monitoring at the supermeter level using transmittance PPGI. For this purpose, a novel PPGI system was designed at the hardware and software level using ambient correction via temporally coded illumination (TCI) and signal processing for PPGI signal extraction. Experimental results show that the processing steps yield a substantially more pulsatile PPGI signal than the raw acquired signal, resulting in statistically significant increases in correlation to ground-truth PPG in both short- ($p \in [<0.0001, 0.040]$) and long-distance ($p \in [<0.0001, 0.056]$) monitoring. The results support the hypothesis that long-distance heart rate monitoring is feasible using transmittance PPGI, allowing for new possibilities of monitoring cardiovascular function in a non-contact manner.